The present invention relates to a noise reduction system for actively blocking a noise source, and to a noise reduction method for actively blocking a noise source, for a means of transport, as well as to the use of a noise reduction system in a means of transport, and to a means of transport comprising a noise reduction system.
In many technical applications, as a result of machine noise, propeller noise or other interference, wave fields are induced in interior spaces, in particular in the low-frequency range (f<500 Hz).
Due to regulations relating to noise protection, or in order to increase the level of comfort, measures for noise abatement are frequently taken. At present a multitude of passive and active measures are available. In particular when applied in the field of low frequencies, active systems make it possible to achieve significant savings in weight and space; however, the effective use of active counter sound systems in interior spaces involves significant expenditure in the positioning of the actuators and sensors (S. J. Elliott: “Signal processing for active control”, Academic Press, San Diego, Calif., 2001).
In many applications the harmonic interference sound, which is mostly stationary, is conveyed to an interior space by way of a limited number of transmission paths. These transmission paths comprise, for example, openings or vibrating structures. As a result of these, sound energy is transported into the interior space, which sound energy is, for example, measurable in the form of various sound values, for example the acoustic power of the acoustic pressure or the acoustic intensity.
Presently, a host of different methods for reducing the acoustic power radiated through a wall are known. Most of the methods are designed such that potential acoustic energy in an interior space can be reduced.
A reduction of the potential acoustic energies essentially requires a global distribution of microphones and loudspeakers. These microphones and loudspeakers are connected to a regulator that controls the loudspeakers such that the sound pressure and thus the potential acoustic energy at the microphones can be minimised. This requires a multitude of different components, for example microphones and loudspeakers, which have to be distributed in a targeted and expensive manner in order to achieve a significant noise reduction not only at a point but also in an entire interior space (S. D. Snyder: “Active noise control primer”, American Institute of Physics, 2000).
For noise reduction in interior spaces it is also possible to damp the vibrating and noise-generating structures, for example the aircraft fuselage (S. D. Snyder: “Active noise control primer”, American Institute of Physics, 2000). The forces which in this process have to be introduced into the vibrating structure can, in turn, reduce the lifespan life of the structure. Furthermore, there are difficulties relating to the control of the actuators necessary for this, because, for example, in aircraft engineering the use of piezoceramic actuators is not possible due to the capacity required and due to the approval conditions that have to be met. Furthermore, it is not inevitable that solely from a vibration amplitude of a vibrating structure deductions about its radiated acoustic performance can be drawn (F. Fahy, P. Gardonio: “Sound and structural vibration”, Academic Press, Oxford, 2007).
Furthermore, the vibrations of the radiating structure can be reduced in that as the field variable to be minimised, the sound pressure at microphones distributed in the interior space is minimised. However, in this context the quality of the actuator technique used and of the distribution of the sensors plays a significant role, wherein implementation is made more difficult (C. H. Hansen: “Understanding active noise cancellation”, Spon Press, London, 2001).
J. Hald in “A power controlled active noise cancellation technique” (International Symposium on Active Noise Control of Sound and Vibration, pp 285-290, 1991) shows a counter sound source in order to absorb the radiated energy of a noise source. However, the success of this method depends on the type of primary source, because additional absorption can cause the primary source to radiate more strongly so that the acoustic pressure level in the interior space is increased.
S. J. Elliott et al. in “Power output minimization and power absorption in the active control of sound” (Journal of Acoustical Society of America 90 (5), pp 2501-2512, 1991) describes a method for minimising the entire radiated output of sound sources and counter sound sources. As a prerequisite for this, the surface velocity of the sound must be known both of the noise source and of the counter sound source, which can result in considerable complexity of the measuring method.